CONCEPTS OF SPACE AND TIME 15 Table 2.3 Main characteristics of the relative space-time view Figure 2.3 (a) New Zealand in absolute space (b) New Zealand in 1953 time space (c) New Zealand in 1970 time space (Reprinted by permission of Oxford Press Source Gatrell 1983, p 111) 16 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS 2.7 CHOOSING THE VIEW FOR A GIS Harvey argues that we ‘have (frequently) assumed a particular spatial language (i.e absolute view or relative view) to be appropriate without examining the rationale for such a choice’ (Harvey, 1969, p 161) After all, we should not discriminate one over another They are complementary As Peuquet (1994) points out, the absolute view requires some sort of measurements referenced to a constant base, implying nonjudgmental observation The relative view, on the other hand, involves interpretation of processes and the flux of changing patterns within a knowledge domain However, a question still remains about integrating absolute and relative views How can we have both perspectives placed in the same representation? Perhaps the answer lies in time geography: ‘Owing to the circumstances under which…[Time Geography] has been developed, its contents, and its applications to date, there is a great danger that Hägerstrand’s time geographic framework will be mistakenly construed as nothing more than a planning tool On the contrary,…the potential usefulness of the framework…is of much greater range’ (Pred, 1977, p 213) Consequently, this book proposes that the concepts of Time Geography should be exploited by GIS to capture the absolute space-time view as well as the relative space-time view The next section provides an overview of Time Geography 2.8 TIME GEOGRAPHY The pioneering work of Torsten Hägerstrand during the 1960s unfolded the Time Geography research that emerged from the Royal University of Lund in Sweden The concepts developed in time geography have been mostly consolidated in the work of Hägerstrand and his students and collaborators Lenntorp, Mårtensson and Carlstein Pred (1977) provides an overview of the main uses of Time Geography in several domains, among them domains concerned with regional development policies, nationwide physical planning, and urbanisation and settlement policies The Swedish government has implemented many of the applications of Time Geography in order to provide adequate job-market opportunities, and a satisfactory level of social and cultural services Some examples are the accessibility simulation of daily individual activities in urban environments and regions (Lenntorp, 1978), comparative studies of living conditions in different populated regions (Mårtensson, 1978), and analysis of various activities in the quaternary sector, mainly concerned with employment distribution (Olander and Carlstein, 1978) Space and time in Time Geography are considered as orthogonal dimensions that become fused into a space-time path representing the trajectory for the lifespan of an entity (Figure 2.4) For simplification, as suggested by Hägerstrand, the representation of space is along only one dimension to maintain the clarity of the proposed representation framework and to give a better visualisation of the evolution of public boundaries The same simplification has been adopted for the time dimension CONCEPTS OF SPACE AND TIME 17 Figure 2.4 Space-time paths of three entities Time is viewed as the dimensional axis that orders events, separates causes from effects, and synchronises and integrates human activity (absolute time representation) Space is viewed as the dimensional axis that represents the changes in the location of an entity in space (absolute spatial representation) Space and time are joined in a single space-time path Space and time are considered inseparable within a path, and it is the timing component which gives structure to space and thus evokes the notion of place Place is ‘a pause in movement’ (Parkes and Thrift, 1980, p 120) The Time Geography approach is an effort to capture the complexity of spacetime interaction at the scale of the smallest indivisible unit in space and time, i.e the space-time path (relative space-time view) However, by having space and time as orthogonal dimensions, Time Geography requires the absolute view of space and time An absolute location in the space axis and an absolute location in the time axis are needed to define a place on a space-time path The absolute location in the space axis may be determined with reference to a coordinate system, whereas the absolute location in the time axis may refer to location in time derived from a clock or calendar A space-time path provides the starting time, the duration, the frequency, the sequential order, as well as the relative location of events and changes that have occurred in a lifespan of an entity The continuous line of a space-time path is the relative space-time representation of the lifespan of an entity It is a descriptive form to reveal the interdependence and relationships between events and changes (Lenntorp, 1976) ‘Space and time are to be jointly treated…because when events are seen located together in a block of spacetime [paths], they inevitably expose relations which cannot be traced if those events are bunched into classes and drawn out of their place in the block, i.e., conventionally analyzed’ (Pred, 1977, p 210) 18 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS Figure 2.5 An example of potential path areas (Reprinted with permission from Parkes and Thrift, 1980, John Wiley & Sons Ltd) The sense of past, present and future depends on where the observer is placed in the space-time path The observer has the awareness of coexistence (connection or togetherness) relationships between space and time at every place located in a spacetime path (Hägerstrand, 1975) The motion of an observer can be limited to a set of circumstances described by the constraints which have been defined for a spacetime path Called the potential path areas (Figure 2.5), this set is represented as a prism in Time Geography It comprises space-time positions for which the possibility of being included in the observer’s trajectory is greater than zero (Lenntorp, 1976) A general procedure cannot be developed for deriving or calculating potential path areas from empirical data Each knowledge domain has to be analysed in order to generate its actual potential path areas CONCEPTS OF SPACE AND TIME 19 2.9 TIME GEOGRAPHY AND GIS Although Time Geography is an effective approach to dealing with space and time in an integrated manner within a GIS, it has so far been neglected After analysing the feasibility of handling space-time concepts of Time Geography within a GIS, Miller affirms that ‘Geographic Information Systems, through their ability to manipulate and analyse spatial data, can allow more widespread use of the space-time perspective [of Time Geography] in spatial modelling and analysis’ (1991, p 300) Few examples are available for illustrating the attempts at applying the Time Geography approach within a GIS Miller (1991) has generated potential path areas (PPAs) for a transportation network application on the basis of arcs in the network that are feasible to travel The mainframe version 5.0 of ARC/INFO was used for the implementation in order to handle a set of nodes and arcs, keep records of locations within the system, and handle numerous travel times at both nodes and arcs in the network Although ‘ARC/INFO NETWORK can meet the requirements for standard GIS applications; it is inefficient and unwieldy in meeting the more specialised needs of the network PPA procedure Whereas ARC/INFO is certainly not representative of all GIS software, it does provide a benchmark which indicates the problems encountered by analysts who wish to use GIS technology in more specialized research and modelling’ (Miller, 1991, p 299) Miller also points out the main requirements in applying Time Geography in a GIS: < The Time Geography approach requires data at a detailed spatial scale in order to obtain an effective analysis in a GIS < The GIS must be able to address the behavioural aspects of data to generate more realistic operational PPAs < The favoured GIS to implement a time geographic framework must be able to store and manipulate topological relationships to avoid adding unnecessary complexity to the framework < The derivation and manipulation of PPAs in a GIS might be accomplished by developing the framework to support space and time constraints A modular structure with inflexibility of key commands and procedures can render a GIS unable to derive the desired space-time prism framework Another example is the application of Time Geography for simulating an individual’s daily shopping behaviour within a GIS (Makin, 1992) The results show how time and space constraints on people’s shopping movements affect shops’ potential earnings and profits Makin explores the potential of using a GIS to structure spatial relationships according to which routes are accessible to each other, and where the buildings are located on the route network He also points out the potential benefits of having implemented his Time Geography model into Smallworld GIS for simulating the behaviour of people’s movements: 20 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS < The data items are not generalised or aggregated within the GIS < The entities are allowed to move and interact with their constraints in space and time in a way that long-term behavioural patterns can be analysed < The model is expressed in terms that not require abstraction into mathematics < The whole system can be organised in a modular fashion in which subsystems are created for reducing the complexity of the model by minimising the amount of data and the number of interactions These two examples illustrate the potential perspectives of applying Time Geography in a GIS And Time Geography could be used to formulate space-time semantic abstractions in the design of a spatio-temporal model based on the object-oriented approach—something not thoroughly explored until now 2.10 MAIN ELEMENTS OF A SPACE-TIME PATH Space-time paths can be defined as image schemata that directly depict the lifespan of each singular entity in a knowledge domain The space-time path is based on the fact that an observer can move anywhere along the space-time path, there is a starting location (in space and time), a direction (motion, change), and a sequence of continuous locations (in space and time) that the observer goes across in following the path The space-time path embodies the structures of identity, location (in space and time) and direction that are the basic abstract metaphors for modelling spatiotemporal data in GIS Langran (1992b) asserts there are at least three sorts of spatio-temporal data in GIS: states, events and evidence In extending this classification, Kraak and MacEachren (1994) point out a further differentiation between events and episodes In essence, a state represents a version of what we know about an entity in a given moment States can consist of different versions of an individual entity (the changes of an individual political boundary marking the variation on the distribution of a territory and its sovereignty) or an ensemble of entities (the changes in ecosystem conditions produced by reductions in atmospheric emissions of pollutants) An event is the moment in time an occurrence takes place Events cause one state to change to another (e.g a cadastral survey may take place due to changes on properties’ boundaries) Events are also part of a process of change caused by action and reaction as well as the synthesis of both (e.g the process of energy propagation and ozone hole formation) An episode is defined as the length of time during which change occurs, a state exists, or an event lasts And finally, a piece of evidence is the datum describing the source of state and event data No evidence should ever be stored in a GIS without referencing its source document, survey or update procedure This book proposes the space-time paths as ideal image schemata for representing and organising the spatio-temporal data categories in GIS (state, event, episode and evidence) The aim is a better understanding of the defined categories by distinguishing the three main structures of a space-time path: identity, location and direction Any CONCEPTS OF SPACE AND TIME 21 Figure 2.6 Possible configurations for a space-time path knowledge domain contains a population made up of entities which are represented in the Time Geography as point objects A singular space-time path describes a lifespan trajectory through time over space, from the point when and where the entity comes into being, to the point when and where the entity ceases to be As a result, a singular space-time path must exist provided an entity exists However, it does not necessarily mean that a space-time path must have a longitudinal linear configuration On the contrary, space-time paths can have either longitudinal or branching configurations (Figure 2.6) Longitudinal configurations imply there is no possibility of having two or more directions over the past or future during a lifespan of an entity They also imply that the exact spatial location of an entity is known at all times Most cases of short-term changes making up the history of day-to-day occurrences of an entity portray longitudinal space-time paths Examples are transport maintenance of state roadways, public works of utility companies, and cadastral measurements On the other hand, branching configurations are encountered in medium-and long-term changes in the lifespan of an entity Examples are the effects of pollution given various climatic and economic scenarios, accident and disease patterns, and land use and demographic trends Each space-time path is in fact the spatio-temporal signature of any entity in a knowledge domain And as such, it has the ability to reveal the connections and interrelations among different entities The identity, location and direction of a spacetime path are fundamental structures to connect and interrelate the spatio-temporal data categories (state, event, episode and evidence) The following sections describe how this can be achieved 2.10.1 State as an element of the space-time path A state can be depicted as a specific location in the space axis, illustrated here using two dimensions for clarity In the lifespan of an entity, changes occur in its space-time path describing the evolution of its inner existence, or a mutation of its location in space Historical evolution regards change as the adaptation of an entity to its 22 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS Figure 2.7 Examples of states as elements of the space-time path Figure 2.8 Co-location in space environment by the process of differentiation and increasing structural complexity (Figure 2.7(a)) Mutation relates change to the theory of revolution, emphasising the importance of conflict or struggle as the principal mechanism of change It characterises any alteration in the direction of the space-time path, hence the change of its location in space (Figure 2.7(b)) States tend to represent the short-term changes which can occur during the lifespan of entities Such changes can occur due to man-made alteration in the position of entities on the ground In contrast, states can also be related to the long-term and medium-term changes such as environmental changes Changes can also occur due to natural causes, and the most common example is displacement of watercourse for rivers and streams In any of these cases the spatio-temporal path gives the co-location of changes in space and time, as illustrated in Figure 2.8 2.10.2 Event as an element of the space-time path Those familiar with event-oriented representation and update-oriented representation should not mistake the concept of event in Time Geography Both representations CONCEPTS OF SPACE AND TIME 23 Figure 2.9 (a) The space-time shows a sequence of events that have occurred over the lifespan of an entity without causing any change in the spatial location of this entity, (b) The space-time path shows the ordering of the events and their respective associated changes over the spatial location of an entity have been discussed as pragmatic solutions for representing spatio-temporal data in GIS (Langran, 1993) In an event-oriented representation, events are described by the moments of change Only the events accountable for some sort of change over a state of an entity are represented The occurrence of an event always causes the creation of a new version from a previous state of an entity Conversely, in an update-oriented representation, events are related to the occurrence of updates of the data stored in a GIS The lifespan of an entity is represented by a sequence of events that represent the occurrence of all kinds of updates, even those updates that are not accountable for any change in the state of this entity These updates usually constitute a resurvey of an entire region, regardless of where or what change has occurred Photogrammetric and remote sensing surveys are examples of collection-driven updates that are scheduled to occur in a given time interval A different perspective in representing events is found in the space-time path described in Time Geography Events are not necessarily related to the creation of versions (states) or update activities in GIS Time Geography emphasises the need for understanding geographic processes in which the mechanisms of change as well as the patterns of change have to be analysed through time The sequence of events through time is viewed as the spatio-temporal manifestation of certain processes The space-time path provides the connection, ordering and synchronisation of events, and their association with the respective changes (states) if they have occurred over a lifespan of an entity (Figure 2.9) In placing events in the time axis of a space-time path, the meaning of an existence and mutation is given to a lifespan of an entity Each space-time path captures the spatial and temporal sequence and coexistence of events An ensemble of space-time paths belonging to several different entities can also represent the interaction among these entities Such an interaction is given by the co-location of events in time (Figure 2.10) GIS can be used to keep the record of ‘co-location of events’ of space-time paths of a number of entities The outcome is a web formed by the interrelations of individual trajectories of several space-time paths Such a web can uncover processes 24 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS Figure 2.10 Co-location in time that are responsible for the ‘temporal connectedness’ of common existence in time Hägerstrand (1975) has previously named these processes as ‘collateral processes’— processes which not unfold independently but are observed from their common existence in time 2.10.3 Episode as an element of the space-time path An episode is defined as the length of time during which change occurs, a state exists or an event lasts The length of the space-time path and its angularity with the time axis are important in classifying changes according to their respective duration: the larger the angle, the shorter the duration Three main types of change can be characterised in a space-time path (Parkes and Thrift, 1980): < Long-term changes modifying the environment < Medium-term changes transforming cultures < Short-term changes making up the history of day-to-day incidents 2.10.4 Evidence as an element of the space-time path An entity is represented as a point object in Time Geography Evidence is the data about entities, events and states that have occurred Each place on the space-time path can only exist if there is a state associated with that place and evidence to confirm an event occurred (Figure 2.11) The space and time axes will determine the scale in which a space-time path occurs In practice this is determined by the spatio-temporal data collected for a GIS 2.11 UNCOVERING SPACE-TIME PATHS The possibilities of defining the events, states and episodes that belong to a spacetime path are immense for any knowledge domain in geographic information sciences CONCEPTS OF SPACE AND TIME 25 Figure 2.1 Evidence as an element of the space-time path Consequently, a space-time path can be constructed according to an ensemble of constraints which define the circumstances what, where and when over space and time These constraints operate on entities, events and states depending on a set of circumstances linked to the individual entity and its environment This involves spatiotemporal data modelling of the behaviour of space-time paths Three types of constraint are described in Time Geography (Hägersrrand, 1975) and can be used in designing the spatio-temporal data model: < Capability constraints limit the trajectories of the space-time paths Space has a limited capacity to accommodate events because entities cannot occupy the same space at the same time, and every entity has a geometric boundary in space Therefore every space has a packing capacity defined by the types of entities to be packed into it Some constraints have a predominant time factor, at rather strictly regular intervals Others can have a dominant space factor, forming bounded regions or volumes < Coupling constraints define where, when and for how long the events and states have to join a space-time path of an entity Coupling constraints can reveal the pattern of space-time paths by exhibiting prism, area or volume configurations known as the potential path areas (PPAs) < Authority constraints impose restricted access to space-time paths The main purpose of defining authority constraints is to organise access to the data as well as to define domains of authority In Time Geography the domain of authority is shown as a cylinder (Figure 2.5) 2.12 CONCLUSIONS This chapter has focused on the main concepts of time geography Many references in the literature introduce the Time Geography approach and its possible applications The reader interested in an expanded coverage of these topics can find a good start with Hall (1966), Karlqvist, Lunqvist and Snickars (1975), Lenntorp (1976), Pred 26 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS (1977), Carlstein, Parkes and Thrift (1978), Parkes and Thrift (1980) and Golledge and Stimson (1997) The main concepts of Time Geography discussed in this chapter play an important role in the design of spatio-temporal data models in GIS Time Geography is not just another theoretical framework but it discloses how the integration between the absolute and relative views of space and time can be devised in GIS Besides, the whole rationale of the Time Geography approach is the inseparability of space and time In other words, states, events, episodes and evidence are all interrelated and connected through a space-time path of an entity The space-time paths provide the image schemata for modelling spatio-temporal data of a knowledge domain in GIS Very little information is available in the published literature on using time geography with GIS This chapter has described two efforts made in this area, Miller (1991) and Makin (1992) The semantics of states, events, episodes and evidence used in this chapter, in order to describe a space-time path, were not related to any particular level of abstraction, be it geographic (nation, region, centre), temporal (year, month, day) or demographic (population, group, individual) The purpose was to provide suitable semantics for modelling spatio-temporal data at multi-scales in space and time; readers may then adapt them to their specific knowledge domains A practical example will be given in Chapter to show how these semantics can be applied and implemented into a spatio-temporal data model based on knowledge domain areas such as historical geography The usefulness of Time Geography lies in providing space-time semantics to objectoriented analysis and design of spatio-temporal data models in GIS Chapter considers the concepts behind the object-oriented approach CHAPTER THREE Object-oriented analysis and design Object-oriented methods cover methods for design and methods for analysis Sometimes there is an overlap, and it is really an idealization to say that they are completely separate activities I.Graham, Object-Oriented Methods This chapter provides a historical background to object-oriented data management, illustrating the diverse efforts involved in object-oriented methods, temporal databases and version management approaches It helps to explain the main concepts in the object-oriented paradigm that are essential for developing a spatio-temporal data model A historical background on object orientation summarises the chronological developments from object-oriented programming languages to object-oriented design methods, and finally to object-oriented analysis methods It can be difficult to choose which object-oriented method to apply in a spatio-temporal data model For integrating the time geography framework within our spatio-temporal data model, the objectoriented analysis and design method proposed by Booch (1986, 1991, 1994) is presented as the best in terms of notation, completeness and technique The temporal database research is reviewed on the basis of concepts and techniques developed to establish appropriate temporal data management support for a spatiotemporal data model Version management approaches are then described, emphasising approaches for ordering and updating versions within a model The version management approach should be chosen so it can be effectively integrated with the spatio-temporal data model In our spatio-temporal data model, versions are deemed to be distinct from snapshot series because they represent states that belong to the space-time path of an entity 3.1 HISTORY OF THE OBJECT-ORIENTED PARADIGM The history of object orientation starts in the early 1960s with the efforts of Dahl, Myrhaug and Nygaard in creating and implementing new concepts for programming 27 28 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS discrete simulation applications By 1965 the object-oriented programming language Simula (Dahl and Nygaard, 1966) had been developed on the basis of the ALGOL-60 language, which was specifically oriented towards discrete event simulation Later, in 1967, the same Norwegian team developed the programming language Simula-67 (Dahl, Myrhaug and Nygaard, 1968), once again an extension of ALGOL-60 It is with Simula67 that the basic concepts which characterise existing object-oriented programming languages were first introduced In particular, Simula-67 introduced the notion of an object class defined by its type and the algorithms necessary to its manipulation It also introduced the inheritance mechanisms through which an object class could inherit the data and the algorithms from other object classes However, it was only after the mid-1970s that the concepts introduced by Simula67 were widely recognised The programming language Smalltalk, a result of the work accomplished by Kay, Goldberg, Ingals and others at the Xerox Research Center at Palo Alto (PARC), has become established as the purest representation of objectoriented concepts In Smalltalk everything is perceived as an object, and objects communicate with each other by passing messages Having its origins in Simula and the doctoral research work of Alan Kay, Smalltalk has evolved by integrating the notion of classes and inheritance from Simula as well as the functional abstractions flavour of LISP.1 There have been five releases of Smalltalk running from Smalltalk-72, launched in 1972, to Smalltalk-80, launched in 1980; the other three releases were launched in 1974, 1976 and 1980 Smalltalk-V and Smalltalk-AT have also been created as dialects from the former Smalltalk developments (Krasner, 1981) Generally, Smalltalk is a complete programming environment, having features such as editors, a class hierarchy, browsers and many of the features of a fourth-generation language (Graham, 1994) Booch puts it like this: Next to Simula, Smalltalk is perhaps the most important object-oriented programming language, because its concepts have influenced not only the design of almost every subsequent object-oriented programming language, but also the look and feel of graphic user interfaces such as the Macintosh user interface and Motif (Booch, 1994, p 474) Several object-oriented programming languages have been developed, most of them having their conceptual foundations based on Smalltalk These attempts have tried to overcome the main inefficiency problems of Smalltalk (e.g the lack of support for persistent objects and unfeasibility of having a distributed multi-user environment) but with the pitfall of compromising the purity and consistency of Smalltalk’s features Over 100 object-oriented programming languages have been developed in the past decade However, as Stroustrup points out: ‘One language is not necessarily better than another because it has a feature the other does not—there are many examples to the contrary The important issue is not how many features a language as, but that the LISP stands for list processing; it was originally developed by John McCarthy in 1958 and more recently it has been used in artificial intelligence work  ... into Smallworld GIS for simulating the behaviour of people’s movements: 20 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS < The data items are not generalised or aggregated within the GIS < The entities...16 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS 2.7 CHOOSING THE VIEW FOR A GIS Harvey argues that we ‘have (frequently) assumed a particular spatial... Such a web can uncover processes 24 OBJECT-ORIENTED DESIGN FOR TEMPORAL GIS Figure 2.10 Co-location in time that are responsible for the ? ?temporal connectedness’ of common existence in time Hägerstrand